Printed circuit boards typically have multiple wiring layers. The number of layers is a function of the complexity of the circuit board, including the density of devices on the board and the configuration of device-to-device connections and device-to-backplane connections. Routing of connections that include crossovers generally require more board layers. A couple of integrated circuits on a single printed circuit board can cause substantial connection routing problems. This wiring can be particularly troublesome when multiple ICs connect to busses that are intertwined, as can occur on a backplane connector.
One approach to the routing problem with respect to multiple RAM chips is the use of so-called mirror-image RAM chips. In this approach, a new part number is created with a pinout configuration that is approximately the mirror image of the original part. This mirror image approach can facilitate routing to a pair of RAMs in the case where one RAM chip is mounted on each side of the printed circuit board, roughly overlaying each other. In particular, one part number is used on the top and the other part number is used on the bottom of the board. In this configuration, the wiring to one RAM is used for both, obviating separate wires to the second RAM, and the attendant wiring difficulty. In a variant of this mirror image approach, some RAM parts are pinned out so that a single part number can be used in this manner by re-defining which data and address bits go to each pin. However, this pin redefinition is transparent to the operation of the RAM.